1. Field of the Invention
This invention relates to the improvement of divalent silver oxide cells that utilize divalent silver oxide as the main component of the positive electrode.
2. Description of the Prior Art
Conventional alkaline silver batteries are known in which the positive electrode of the alkaline cells are formed mainly of divalent silver oxide and the negative electrode are mainly formed of zinc or another similar substance. The divalent silver oxide has a large oxygen content as compared with mono valent silver oxide, and thus a cell using divalent silver oxide displays a high discharge capacity. However, since divalent silver oxide displays a poor stability in the cell and will gradually decompose into mono valent silver oxide, during storage of the battery system the oxygen which is evolved within the cell will increase the inner pressure within the cell so much that the cell will deform, or in extreme cases break open. This decomposition of the divalent silver oxide (otherwise called "self-discharge") also means that the amount of divalent silver oxide available for the operation of the cell will decrease, making it impossible for the cell to generate the amount of electrical current originally intended. Particularly in alkaline cells intended for long duration service, this phenomenon of self-discharge poses an important problem.
It is believed that the tendency of the divalent silver oxide in the divalent silver oxide cell to decompose is ascribable to the high activity of the divalent silver oxide and to the consequent reaction of the divalent silver oxide with the alkaline electrolyte with which it is held in direct contact. One method of avoiding this drawback, wherein the surfaces of the divalent silver oxide particles are covered with silver plumbate, is disclosed in U.S. Pat. No. 3,017,448. This method has resulted in an improved storage life for a divalent silver oxide cell.
As a criterion of the stability of divalent silver oxide for use in cells, the value called "gassing rate" or "rate of oxygen evolution" can be used. This criterion is expressed by the volume of oxygen evolved during a fixed length of time by 1 g of divalent silver oxide kept in an aqueous 40% sodium hydroxide or potassium hydroxide solution at 40.degree. C. It should be noted that these conditions are harsher than those under which a divalent silver oxide cell is normally operated.
A divalent silver oxide cell normally uses sodium hydroxide as the electrolyte. For applications which need a large electric current at once, however, these cells require the use of potassium hydroxide as the electrolyte. The conventional divalent silver oxide generally exhibits less stability in the presence of potassium hydroxide than in the presence of sodium hydroxide.
The present inventors, in looking for ways for stabilizing a divalent silver oxide cell having divalent silver oxide in the positive electrode, experimented with the incorporation of various additives to the cell. One discovery was that the addition of a lead source and an aluminum source to either the divalent silver oxide electrode or to the alkaline electrolyte in the divalent silver oxide cell served to give a high stability to the divalent silver oxide. This discovery has been described in detail in Japanese citations JA-OS No. 9222/1981 and JA-OS No. 9966/1981.
An object of this invention is to provide a divalent silver oxide cell having a positive electrode formed mainly of divalent silver oxide wherein the divalent silver oxide will be characterized by a notably improved stability.